Cryptostomata (singular: cryptostoma) are anatomical structures found in some species of Fucus and not in Pelvetia canaliculata. They are conceptacles, sterile cavities, producing only hairs.They are found on the lamina of Fucus serratus. Colourless tufts of hairs can be seen issuing from them.
References
Cryptostoma Wikipedia(Text) CC BY-SA